On alpha-helices terminated by glycine. 1. Identification of common structural features

About one third of all helices is terminated by residues with a positive torsion angle phi. 74% of them are glycines. This strong propensity can be explained by typical bifurcated three-center hydrogen bonds which are only compatible with a positive torsion angle phi, causing helix termination. An algorithm was developed to identify these structural features in alpha-helices. 158 out of 456 helices in 79 different well-refined protein structures examined in our analysis were found to have a glycine with this special conformation which have been conserved remarkably during evolution.